Regarding the lab from Friday on how much heat the water absorbed when turning from ice to boiling water, there are three stages. The first stage is the melting of ice into water.the formula is q=ml. q is the heat, m is the mass and l is the latent heat value. The second stage is the water heating up which is the original q=mcΔt. c is the specific heat capacity and Δt is the change in temperature. There is a latent heat value for fusion (solid to liquid) and vaporization (liquid to gas). The negative values are used if it is the opposite (gas to liquid/liquid to solid).
To summarize, q=ml and latent heat values are used only for phase changes (change of state). Negative latent heat values are for phase changes towards solids.
THIS IS TESTABLE
Heat of a reaction is change in ENTHALPY. Change in enthalpy is the amount of heat produced or used during a chemical reaction. It is ΔH and has units of kJ/mol. ΔH is also equal to the different in enthalpies of products and reactants. Sometimes ΔH will have a subscript to indicate the type of reaction. ΔHc for combustion.
There are multiple ways to calculate ΔH 1. using the value of q and the number of moles 2. using Hess's law 3. using bond energies 4. Using standard heats of formation 5. Potential Energy Diagram
ΔH for exothermic reactions will be negative and endothermic reactions will be positive.
Using standard heats of formation. The value of ΔH can be calculated using the standard ethalpies of formation for the products and reactants. The standard enthalpy of formation (H°subscript f) is the energy associated with making a substance from its elements. Elements of a H° of 0 (O2 or H2) the values for compounds can be found on a table. A balanced chemical equation with states is need to calculate ΔH. ΔH = ɛnH°f products - ɛnH°f reactants
To summarize, q=ml and latent heat values are used only for phase changes (change of state). Negative latent heat values are for phase changes towards solids.
THIS IS TESTABLE
Heat of a reaction is change in ENTHALPY. Change in enthalpy is the amount of heat produced or used during a chemical reaction. It is ΔH and has units of kJ/mol. ΔH is also equal to the different in enthalpies of products and reactants. Sometimes ΔH will have a subscript to indicate the type of reaction. ΔHc for combustion.
There are multiple ways to calculate ΔH
1. using the value of q and the number of moles
2. using Hess's law
3. using bond energies
4. Using standard heats of formation
5. Potential Energy Diagram
ΔH for exothermic reactions will be negative and endothermic reactions will be positive.
Using standard heats of formation. The value of ΔH can be calculated using the standard ethalpies of formation for the products and reactants. The standard enthalpy of formation (H° subscript f) is the energy associated with making a substance from its elements. Elements of a H° of 0 (O2 or H2) the values for compounds can be found on a table. A balanced chemical equation with states is need to calculate ΔH.
ΔH = ɛnH°f products - ɛnH°f reactants
H°f must be multiplied by its molar coefficient.